The invention relates to a capacitive distance measuring system and in particular a capacitive length measuring system for electronically detecting and displaying a position or a displacement distance.
A distance measuring system is known from EP 0 442 898 B1, which system uses a capacitive sensor designed as differential capacitor, as well as a sigma/delta converter, consisting of an integrator and a comparator. The differential capacitor comprises two side-by-side arranged transmitting electrodes and a joint receiving electrode, which is arranged a short distance from the transmitting electrodes on a scale, so as to be displaceable relative to these transmitting electrodes. Depending on the displacement position, the receiving electrode overlays the individual transmitting electrodes differently, which leads to changes in the capacitances for the partial capacitors of the differential capacitor. These changes represent a measure for the displacement distance to be measured.
In order to determine the partial capacitance ratio, the individual transmitting electrodes of the differential capacitor are fed during each cycle with charge packets of a predetermined size but different polarity. A positive charge packet is transmitted to the one transmitting electrode, if the total charge amount transmitted via the receiving electrode to the integrator and integrated therein is negative. A negative charge packet is transmitted to the other transmitting electrode if the total charge is positive. The number of respectively transmitted packets is detected with associated interpolation counters. The ratio of these counters relative to each other reflects the sought after ratio of the partial capacitances.
Several transmitting electrodes are arranged in a row for the measuring of longer lengths. If the measuring range that is fixed by one pair of transmitting electrodes is abandoned during the displacement, then a change to the next pair occurs. The number and direction of the change operations are also counted and are used jointly with the interpolation counters for determining the distance traveled.
The known distance measuring system has proven itself in practical operations. However, interference signals resulting from the triggering of transmitting electrodes, the display and other sources of interference are also integrated and lead to non-linearity, reflected in the measuring accuracy. In addition, a supply source must be provided that transmits charge packets with a specific polarity at the selected moments to the corresponding partial capacitor.
Starting with this premise, it is the object of the invention to create a suitable distance measuring system, in particular for battery-operated hand-held devices, which ensures the highest possible measuring accuracy with the highest possible resolution. In addition, the distance measuring system should operate with a simple energy supply.
The distance measuring system is provided with one or several capacitive sensors, which are designed as differential capacitors. The capacitive distance measuring can be realized with power savings compared to prior methods. The capacitances used are in the pF (pico-farad) range, as a result of which the transmitted charge amounts and the resulting currents are very low. Each sensor comprises at least 2, for example 16, transmitting electrodes that are preferably arranged in a row with a constant distance there between. The sensor further comprises one or preferably several, for example 64, counter electrodes that are also arranged in a row. The electrodes are arranged on a measurer, such as a ruler or measuring tape, opposite the respective transmitting electrodes and at a short, equal distance to these. The measurer is positioned such that it can be adjusted and in particular can be displaced relative to the transmitting electrodes. Thus, at least two partial capacitors are formed in this way, wherein the capacitance of at least one of the partial capacitors changes proportional to the distance distance.
A triggering device is used to feed a binary signal to selected one or groups of transmitting electrodes for generating individual measuring signals at predetermined times. The signals for triggering the two partial capacitors are fixedly offset in phase relative to each other, so that the resulting measuring signals do not overlap in time if possible. If voltage pulses are used for the triggering, the measuring signals essentially are the transmitted current surges that form a charge amount (a charge packet). If current signals or charge packets function as trigger signals, however, voltages are measured.
A switch unit is advantageously provided, containing switches that are closed for a predetermined interval to define time windows. The time windows include selected edges of the trigger signals positioned therein. Inside the time windows, the associated measuring signals are allowed to pass through to a processing device, which evaluates these signals and uses the results to determine the partial capacitances of the differential capacitor. The processing device determines the distance to be measured from the partial capacitances.
The energy supply can have a very simple design. The trigger signals transmitted to the one partial capacitor can have the same curve shape as the trigger signals for the other partial capacitor. They can even have the same polarity and assume the same values, so that positive or negative rectangular signals, for example, are particularly suitable for use as trigger signals. Simple battery cells are sufficient for the energy supply.
It is advantageous if the phase offset and the time window are fixed such that only one selected signal edge can be observed inside one time window. With a 90xc2x0 phase offset, an equidistant spacing of one fourth of the clock cycle is obtained between the edges of the trigger signals for the two partial capacitors. The time window is smaller than the pulse duration of a trigger signal. It is advantageous if the time window essentially includes only the respective edge change, meaning it represents only a fraction of the total signal period. Interference signals consequently do not influence the evaluation during the complete signal duration, but only briefly during the edge change where the interference distance is high. As a result, the linearity of the measuring system is improved and a high measuring accuracy is possible.
The processing device can contain a capacitive sigma/delta converter that essentially consists of an integrator with a downstream-connected comparator. The integrator integrates the charge received from the differential capacitor. The comparator provides an output signal that corresponds to the mathematical sign for this charge and determines the signal edge to be evaluated next. If both partial capacitors are triggered with positive rectangular signals, for example, either the rising edge of the trigger signal for the one partial capacitor or the decreasing edge, offset by half a cycle, of the trigger signal to the other partial capacitor is selected in dependence on the comparator output. With trigger signals having different polarity, either the front or the rear edges of the trigger signals are always evaluated.
The time window should be sufficiently large, so that the received charge packets are almost completely integrated in an integration capacitor of the integrator, taking into consideration the internal resistance of the source supplying the integration capacitor. The time window preferably is larger than the sum of the values for the trigger signal rise time and ten times the time constant xcfx84=RiC, where Ri characterizes the internal resistance of the voltage source used and C the maximum possible measuring capacitance. On the other hand, the time window should be as small as possible, meaning it should essentially only detect the signal edges and should be synchronized as accurately as possible with these edges, to limit the influence of non-linearity and interference signals as much as possible.
The switch unit according to one embodiment of the invention also makes it possible to create defined voltage conditions at the partial capacitors of a differential capacitor. The partial capacitors are in principle polarized relative to each other before a trigger signal is transmitted, without this changing the integrated total charge. The required measuring system linearity for a high measuring accuracy is ensured in this way.
The measuring system operation is synchronous, meaning the transmitting signals and the measuring signals are controlled by the same clock generator. As a result, interference signals are for the most part suppressed. The influence of low-frequency interference caused by the activation of the display unit, for example a liquid-crystal display (LCD), or by interference signals induced on the scale can be reduced further if the transmitting and measuring signals are inverted periodically after a number of clock cycles, preferably after each clock cycle. This can be realized through a periodic switch during the evaluation from the one edge of the measuring signals to the other edge.
The partition of the counter electrodes is proportional to that of the transmitting electrode rows. The width of the counter electrodes, measured in displacement direction, advantageously corresponds to a whole-number multiple of the width of the transmitting electrodes. This permits large areas of overlay with correspondingly large partial or measuring capacitances, a simple evaluation and long rows of counter electrodes with correspondingly large measuring ranges. In addition, each second counter electrode can be connected to ground because several transmitting electrode pairs arranged side by side can be assigned to an active counter electrode. As a result, even larger scale increments, meaning interpolation periods, and thus even larger absolute measuring distances can be achieved. However, it is also possible to provide only a few counter electrodes and several transmitting electrodes.
The signals transmitted to the counter electrodes are tapped via friction or non-friction contacts, for example via a receiving electrode with capacitive coupling to the counter electrode. The receiving electrode can be arranged together with the transmitting electrode row on a joint sensor head, for example parallel to it. To avoid cross talk, it is advantageous if this electrode is insulated against the transmitting electrodes, for example via grounded screen. Connecting lines need only extend to the sensor head and movable contacts can be omitted.
The partial capacitances are determined through interpolation. For this, the processing unit has a counter unit with two counters, which can each have a depth of 8 to 16 bits , preferably 8, 10 or 12 bits, depending on the desired resolution. One or several pulses are sent to each partial capacitor and the resulting charges are integrated. If a measuring signal is evaluated in one of the partial capacitors, the associated counter is incremented. The counter that first reaches its maximum value is used as denominator for computing the interpolation value. The other counter determines the interpolation value.
The counters are furthermore used to determine the pair of transmitting electrodes, which must be triggered and is sufficiently overlaid by the counter electrode. Few interpolation errors occur if the degree of overlay is always within a specified range, for example between 1:2 and 2:1. It is possible to detect whether the decree of overlay is out of this range in that one of the counters is incremented several times successively.
The transmitting electrodes can be combined into groups of several, for example 6, electrodes that offer a corresponding number of triggering options. Within the ranges fixed by the groups, the suitable electrode pair is triggered and the counter settings determined. Stringing together transmitting electrodes can increase the scale increments.
The distance measuring system of one particularly advantageous embodiment of the invention is designed as absolute distance measuring system. That is a system which supplies at any point in time the desired absolute position using only the currently detected values. A constant reading for control during the adjustment is not necessary as well as a readjustment of zero state following each start-up is not required. The active period can be reduced considerably and the measuring system can operate with low cycle rates, thus making the design extremely power saving. As a result, it is particularly suitable for battery-operated measuring devices. In addition, the susceptibility to errors can be reduced further because the measuring system does not lose the zero point due to an excessively high traversing speed or due to contamination.
The absolute measuring system according to the invention is provided with at least two capacitive sensors, designed as differential capacitors, which form two measuring systems with different partitions. In both measuring systems, the ratios of the partial capacitances of the respective differential capacitor, relative to each other, are determined. The absolute position can be determined from that.
A single receiving electrode only is used for one advantageous, space-saving arrangement. This receiving electrode is arranged between the rows of transmitting electrodes and parallel to these in the absolute distance measuring system. The counter electrodes are connected element-by-element to form a single row. Alternating in time, the transmitting electrode rows can also function either as a transmitter or as a receiver for the other transmitting electrode row. This kind of arrangement allows for the largest amount of surface area for the capacitor plates on a given surface of the sensor head and thus a good capacitive coupling.
A separate processing unit can be assigned to each capacitive sensor, so that the evaluation is synchronized as much as possible. The sigma/delta converter, however, can be used jointly by both sensors if a separate integration capacitor that can be added is provided for each sensor. A linking unit uses the resulting relative positions for determining the absolute position. However, it is also possible to have applications where a joint processing unit performs a serial evaluation of the measuring signals from both differential capacitors. To ensure a synchronous operation, the individual differential capacitors can be assigned buffer units, which are charged up with one or several charge pulses and synchronous if possible. The output voltages from these buffer units can be evaluated successively and slowly.
The distance measuring system according to the invention permits the detection of relatively large measuring distances with a high resolution and low interpolation errors. If necessary, the measuring range can be expanded nearly optionally. One simple option consists in reducing the width of a counter electrode row in sections and crosswise to the displacement direction. The width of the other counter electrode row can remain the same or can even be widened in sections. In any case, specific capacity ratios between the rows of transmitting electrodes and counter electrodes result in each section.
The switches, capacitors and operational amplifiers required for the evaluation can be realized with standard MOS technology, which is technically advantageous and cost-effective. The counter, comparative and other operations can be realized advantageously in a correspondingly programmed microcontroller or processor, even though a realization with circuit engineering is possible as well.
All past embodiments related in particular to a length measuring system. However, the distance measuring system is also suitable for the angle measuring. For that, the transmitting electrodes can be formed on the outside of a circular disk and the counter electrodes on the inside of a ring-shaped material measure, at a short distance to the disk and arranged such that they can rotate around their center axis.